Method of assisted oil recovery



May 24, 1966 P. E. BAKER ETAI.

METHOD OF ASSISTED OIL RECOVERY Filed OCT.. 22 1965 3 Sheets--Sheet 1 May 24, 1966 P. E. BAKER r-:TAL

METHOD OF ASSISTED OIL RECOVERY Filed Oct. 22, 1963 3 Sheets-Sheet 2 OIL SOLUBLE GAS INJECTED AT P1 l0 ATM. P

J*VISCOSITY DETECTOR OIL BEARING FORMATION 2 4 INVENTORS PAU/ E. BAKER GEORGE D. GOULD FIGA May 24, 1966 P. E. BAKER ETAL 3,252,512

METHoD 0F AssIsTED on, RECOVERY Filed Oct. 22, 1963 5 Sheets-Sheet 3 j 2 1000 o (L l z 800 200 gg m o u. 600 /5 :i V) *P o U U 2 g3 400 10o O l 200 so n z n A 0 0 w PRESSURE, POUNDS PER SQUARE INCH ABSOLUTE FIG.5

INVENTORS PAUL E. BAKER GEORGE D. GUULD 3,252,512 METHOD OF'ASSISTED OIL RECOVERY Paul E. Baker, Fullerton, and George D. Gould, Orinda,

Calif., assignors to Chevron Research Company, a corporation of Delaware Filed Get. 22, 1963, Ser. No. 328,769 1 Claim. (Cl. 16d- 2) This application is a continuation-in-part of application Serial No. 860,265 filed December 17, 1959, and now abandoned.

The present invention relates to assisted oil recovery methods. More particularly, it relates to a method of i-mproving oil recovery during an assisted recovery project, such as a waterliood for example, by locally stimulating oil production from a producing well in an oil field being water-flooded. Still more particularly the present invention provides for injecting an oil-soluble substance through a production well to reduce the viscosity of the oil around the well, and then alternately producing the less viscous oil from this well, while applying a driving force at a distant well, and then again injecting an oil-soluble substance through the production well as the viscosity of the oil in place approaches the original viscosity and alternately producing the oil and reducing its viscosity.

It is a particular object of the present invention to increase the production of oil from an oil-bearing formation, which formation is undergoing conventional assisted recovery operations and wherein the oils viscosity is so relatively high that it does not satisfactorily lend itself to recovery by more conventional methods by providing for reducing the viscosity of the oil adjacent the producing well during the course of the waterflood.

The method of the invention is particularly well adapted to the recovery of very heavy crudes such as are commonly found in the California and Pennsylvania elds. Although the term heavy oil is only relative nomenclature and may be defined differently in different localities, the method of this invention would be the same in all areas and does not depend on local definition. It is contemplated that the process would have its greatest adaptation in recovering oils with a gravity of less than 20 API. The method is useful in partially depleted formations having a free gas phase throughout the formation. Thus the method of this invention is not practiced in formations having no free gas phase or in formations having a gas cap. The method of this invention is not limited to use only with waterooding. It may be utilized in combination with other types of assisted recovery.

A major problem, heretofore unsolved, has been the economic recovery of relatively viscous oil from partially depleted reservoirs. The resistance to oil iiow into a producing well primarily occurs immediately around the well. The methods heretofore used, such as waterooding or gas repressuring utilized the driving force of water or gas injected into the formation at injection wells to drive oil into the production wells. These methods did not provide for improving the oil flow in the critical zone immediately around the production wells.

The presentl invention provides a method of locally stimulating production from the wells in a eld in which a conventional assisted recovery project is under way. In accordance with the preferred method of carrying out the present invention, a waterflood is initiated in a field, an oil-soluble substance such as an oil-soluble gas is injected through a producing well under pressure so that it is caused to go into solution with the oil in the oilbearing formation immediately around the well. I'he injection pressure is adjusted to cause substantially all the gas to dissolve in the oil and in the water immediately around the well. As the gas goes into solution in the oil, it effectively reduces the viscosity of the oil immedinitcd StatesA Patent O 'ice atcly around the well. Since in oil production a very high percentage of the resistance to flow is in a region near the producing well, reduction of the viscosity in this zone alone is sufficient to permit significantly improved recovery of the viscous oil.

After a predetermined amount of gas has been injected into the formation, injection of gas is stopped and oil is then produced through the well. The amount of gas which is injected into the formation through the producing well is limited so that an excessive gas saturation in the vicinity of the producing well will not prevent the oil from flowing into the well after gas injection is stopped.

A waterood is initiated at other wells in the formation and aids in moving the less viscous oil into the well previously used for gas injection. Since the viscosity of the oil has been reduced by the gas injection, this is much more easily accomplished. Thus the gas injection well becomes a producing well. During production a sufficient back pressure is maintained in the producing well to prevent excessive loss of gas from the oil in the formation. Recovery is continued until the viscosity of the oil in the vicinity of the producing well again becomes undesirably high. At this time, if economically feasible, additional oil-soluble gas is injected through the producing well to again decrease the oils viscosity. Thus, a cyclic process is set up wherein an oil-soluble gas is injected to lower the viscosity of the oil and then the` reduced-viscosity oil is recovered through the same well until the viscosity of the oil approaches the original viscosity, at which time more oil-soluble gas is injected into the formation and then recovery of oil is continued. Further objects and advantages of the present method will become apparent from the following detailed description read in view of the accompanying drawings which are a part of the present specification and in which:

FIGURE 1 is a schematic representation of the vertical section of a typical oil-bearing formation penetrated by a water injection well and an oil production well wherein an oil-soluble substance has been injected under pressure sufficient to cause the substance to go into solution with oil laround the production well thereby setting up a zone of relatively less viscous oil.

FIGURE 2 is a schematic representation of the same vertical section wherein an assisted recovery drive, in Ithis particular instance a waterood, has been initiated through the water injection well, and relatively less viscous oil produced under back pressure at the oil production well.-

FIGURE 3 is a schematic representation of the same vertical section wherein oil-soluble gas is again injected into the formation through the production well.

FIGURE 4 is a schematic representation of the same vertical section wherein oil is again produced under back pressure through the oil product-ion well while the assisted recovery drive initiated through the water injection well is continued.

FIGURE 5 is a graphical representation showing the viscosity and gas content of oil as a function of pressure.

With `reference to the drawings and to FIGURE 1 in particular, an oil-bearing formation 24 penetrated by a producing well 20 is shown. In this regard the present invention is useful in recovering additional `oil from par,- tially depleted formations having a free gas phase substantially throughout the formation. This is an important feature of the present invention because it allows the oilsoluble `gas which is injected into the formation to readily contact and to go into solution with the oil surrounding the well. The formation in which the method of the present invention is utilized is in direct contrast with the type of formation that has a gas cap and is under i high pressure.

ln accordance with the present invention an oil-soluble substance, such as for example an oil-soluble fluid or gas, such as carbon dioxide or a mixture ol' light hydrocarbons, is injected into the formation. Other oil-soluble substances useful in the present invention include methane, ethane, propane, butane, ethylene, propylene, acetylene and vhydrogen sulfide, and also mixtures of these gases. The oil-soluble fluid is injected through production well 20 as shown by arrows 22 through a valve 16 into the formation 24 under pressure sufficient to promote the Huid going into solution with the oil in a zone immediately around the production well. The fiuid or gas used in this method may be initially at an elevated pressure. is not sufficient to effect solution, a compressor 14 is used to raise the gas pressure to the desired amount. The optimum injection pressure will, of course, vary from formation to formation depending upon local conditions. The uid or gas may be injected under an initial high pressure or may be initially injected at a relatively lower pressure with the pressure thereafter being gradually increased until the optimum pressure is reached. The gas which has been injected into the formation is now held under a back pressure, determined by the local characteristics of the formation, which is sufficient to hold the gas in solution with the oil.

As shown in FIGURE 2, an assisted recovery drive, for example a waterfiood, denoted by arrows 23, has been initiated through the water injection well 12 by moving water with a pump 26 through a valve 10, then down into the formation 24. Of course the waterflood can be started before injecting the oil-soluble gas at any given production well. In determining the type of assisted recovery in a particular case, it is important that the type of assisted recovery utilized be such that there will be no possibility that any of the components of the assisted recovery drive will tend to rob the oil-gas solution of the gas in the solution. If this stripping of the gas from the oil-gas `solution should occur, the advantage gained in reducing the viscosity of the oil would be lost. It becomes apparent, therefore, that a substance in which the previously injected gas is more soluble than in the oil should not be used as a medium for driving the oil in formation toward the producing well. Although some of the oil-soluble substances useful in the present invention are slightly water-soluble, a waterood is the preferred type of assisted recovery drive.

The relatively less viscous oil can now be produced under back pressure through the production well 20. This production is accomplished by a conventional sucker rod string 28 and lproduction tubing through valve 18. Oil could be produced, however, in a number of different ways well known in the art. The production is carried on until the viscosity of the solution `approaches the original viscosity of the oil. The back pressure held on the oil-gas solution in the formation is adjusted to retain a substantial amount of the gas in solution with oi] in the formation around the `producing well. Some gas will come out of solution as the oil moves from the formation into the production well. However, this is not serious since once the oil is in the well, the need for low viscosity is not so critical.

FIGURE 3 shows the formation after oil vhas been produced and when the relative viscosity of the oil remaining in the zone of injection 22 has approached the viscosity of the oil in the original formation 24, thereby necessitating the injection of additional oil-soluble gas into the formation. This injection is accomplished in the same manner as the original injection of the gas. Whether or not the oil-soluble gas is again injected into the well is a matter of economics and is determined for each well or field individually.

Referring now to FIGURE 4 where a formation is illustrated wherein the zone of relatively less viscous oil has been re'established by injection of gas as described heretofore, the assisted recovery drive continued, and the If this pressure less viscous oil again produced under hack pressure sufl`- cient to keep substantial gas in solution with oil in the formation. Note that after production of the less viscous oil the gas in solution can be recovered from the oil and reused by any one of a number of processes. The feasibility of this reuse depends upon the economics of the particular area of production.

FIGURE 4 also illustrates one method for automatically controlling this eyclic process. lIn this illustration, viscosity detector 34.is attached to the oil producing line 28 in conjunction with pressure gauge 36, pump 26, and valve 18. So long as the viscosity of the oil-gas solution remains below a predetermined value the oil will be produced through line 28 aided by the assisted recovery drive continued through well 12 by means of pump 26. When the viscosity of the oil-gas solution approaches a predeterminable viscosity, the viscosity detector acts through a control system indicated by dashed lines 30, 32, 38, and 40 to deactivate sucker rod string 28 to stop the production of oil, to close valve 18, to open valve 16 to allow gas to ow into the formaiiom After a predetermined amount of gas, as measured by a dow ymeasuring device in valve 16, has been forced linto the formation, the viscosity detector through the control system activates pump 26, valve 18 through pressure gauge 3,6 and water pump 26, closes valve 16, and Oil is again produced through line 28.

In accordance with the present invention the oil in a localized section of a formation immediately surrounding a production well is contacted with an oil-soluble substance. In a preferred embodiment of the invention sulfieient oil-sol-uble gas is injected into a producing1 well to dissolve in the oil present in a 25-ft. radial pattern around the well. That is to say, that depending on temperaturel and pressure, only enough gas is injected to saturate the oil in the formation with a 25-ft. radius of the well. In some instances this radius may be yas much as 40 feet. If more gas is injected .than is required to saturate the oil within a 40-ft. radius however, the advantages of the present method are lost for the reasons advanced above. The minimum radial amount of formation treated is usually about 5 feet in order to obtain the benefits of the invention.

A preferred oil-soluble substance to stimulate wells in accordance with the invention is carbon dioxide. When carbon dioxide is used the upper limit of carbon dioxide gas injected into the formation to obtain the advantages of the invention is dependent on the solubility of carbon dioxide in the formation crude at the pressure and temperature of the formation. For example, the type of formation in which the present invention is primarly useful is a relatively low pressure partially depleted reservoir having a free gas phase. Pressures from near atmosphere to about 1000 p.s.i. are commonly encountered. The present method is probably most widely used at pressures between to 500 p.s.i. and at temperatures from 80 F. to 140 F.

When using carbon dioxide in the invention not more carbon dioxide is injected into the formation than will dissolve in the oil present in a 40-ft. radius from the production well. Preferably not more carbon dioxide is injected than that which will dissolve in the oil in a 25-ft. radius from the injection wel-l. After this amount of carbon dioxide isinjected the oil in which the carbon dioxide has dissolved is produced under back pressure which will maintain a substantial amount of carbon dioxide in solution with thel oil remaining in the formation. The carbon dioxide is removed from the oil in the well or at the surface and may be reused in the process.

For example, in using carbon dioxide in accordance with the present invention in a partially depleted oil-bearing formation having a temperature of about F. and having a free gas phase and a pressure of about 100 psi., the porosity of the formation and thc oil saturation thereof must first be determined. If the porosity is .10 and the oil saturation .4 it requires about .2 s.c.f. of carbon dioxide to saturate the oil in a cubic foot of the formation. lf the oil saturation is .8 and the porosity .3, then under the same conditions 1.25 s.c.f. of carbon dioxide are required to saturate the oil in a cubic foot of the formation. With these figures the preferred amount of carbon dioxide and the upper limit of carbon dioxide for the two hypothetical formations can be calculated. Thus in the first instance if the formation is 40 feet thick and it is desired to treat 25 feet of the formation, the s.c.f. of carbon dioxide required is 252 40 .2=15,800 s.c.f. The maximum amount of carbon dioxide that can be injected into the formation and retain the benefits of the invention is 402 3.14 40 .2=40,000 s.c.f.

in a formation having a pressure of 500 p.s.i., and a temperature of 120 F., different limits apply. 1f for example, the porosity of the formation is .l0 and the oil saturation .4 then 1.0 s.c.f. of carbon dioxide will dissolve in the oil in a cubic foot of the formation. If on the other hand the porosity of the formation is .3 and the oil saturation .8, then 6.0 s.c.f. of carbon dioxide will dissolve in the oil in one cubic foot of formation. From these figures the total gas injection can be determined.

If a formation is at a pressure of 1000 p.s.i. and a temperature of 120 F. and has a porosity of .1 and an oil saturation of .4, then 3.0 s.c.f. of carbon dioxide are required per cubic foot of formation. If the porosity is .3 and the oil saturation .8 then 19 s.c.f. of carbon dioxide are required for each cubic foot of formation treated. While it is recognized that there are differences in the solubility of carbon dioxide in various crudes, the above examples are for typical crudes in which the method of the present invention would have its optimum use and therefore have a degree of accuracy within acceptable limits.

A typical example of the method of the present invention in which a mixture of methane and ethane is used in a t0-ft. thick formation having .3 porosity, .1 gas saturation and .5 oil saturation is now set out. The formation is 1000 feet below the surface and will have a pressure of 500 p.s.i. and a temperature of about 100 F. It is desired to treat a 25-ft. radius from the producing well and therefore there are approximately 2000 barrels of oil to be treated. One hundred s.c.f. of the methane-ethane mixture will dissolve in each barrel of oil at formation conditions, therefore 200,000 s.c.f. of the gas are required for the treatment. This treatment provides a 50% reduction in viscosity and therefore approximately doubles the ow rate of the treated oil. If the well were producing 10 barrels of oil per day before the treatment, the production after treatment would be approximately 20 barrels of oil per day with no substantial increase in the Water cut. The treatment would be effective for about 1 to 2 months.

1t willv be apparent to those skilled in the art that the present system permits an appreciable increase in the efcency of the assist-ed recovery from partially depleted t oil-bearing formations. Various changes and modifications-may be made in the method without departing from the spirit of the invention. All such modifications and changes coming within the scope of the appended claim are intended to be included herein.

What is claimed is:

The method of recovering oil from an oil-bearing formation penetrated by a plurality of wells comprising the steps of injecting a predetermined amount of oilsoluble gas through one or more of the wells under sufcient pressure to cause said gas to enter the formation and to go into solution with the oil in said formation setting up a zone of relatively less viscous oil in said formation around said one or more wells, said predetermined amount of oil-soluble gas being not more than will theoretically saturate the oil within a 25-fooft radius of each of said one or more wells, holding sufficient back pressure on said one or more wells in said zone to keep a substantial portion of said gas in solution with said oil in said formation, injecting water through the remaining wells under sucient pressure to eifect a uid flow in the direction of the zone of relatively less viscous oil, and alternately halting said gas injection while recovering the less viscous oil through said one or more wells at a back pressure sufficient to keep a substantial portion of said gas in solution with said oil in said formation until the viscosity of the oil ap proaches the original viscosity and then halting said oil recovery while again injecting oil-soluble gas through said one or more wells into said formation thereby reducing the viscosity of the oil in said zone.

References Cited by the Examiner UNITED STATES PATENTS 1,899,497 2/1933 Doherty 166-9 2,964,109 12/1960 Martin 166-43 3,064,728 11/ 1962 Gould 166-11 X 3,120,262 2/1964 Archer 1669 CHARLES E. OCONNELL, Primary Examiner.

S. J. NOVOSAD, Assistant Examiner. 

